1639698 - font-display fails for first text with line-height

Status: RESOLVED FIXED

(Core :: Layout: Text and Fonts, defect)

Description

[tjeu.kayim]

Attachment: font-block-inline.html

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

Create a document containing several text elements, apply CSS to the body "line-height: 1" and a @font-face with "font-display: block".
See the HTML attachment for a minimal demo.
Tested in Firefox 72.0.1 and Nightly 78.0a1 (2020-05-19).

Actual results:

The first visible text element is showing the fallback font for a brief moment, while the other elements stay hidden until the font is downloaded.

Expected results:

The first element should render the same as the others and adhere "font-display: block".

Comment 1

[Cameron McCormack (:heycam)]

Here's the same test pointing to a font URL that waits 5 seconds before responding, to make the issue more obvious.

Comment 2

[Cameron McCormack (:heycam)]

At the time we render the first line in the fallback font, its text run has mSkipDrawing = false, while the other two text runs have mSkipDrawing = true.

Comment 3

[Cameron McCormack (:heycam)]

Ultimately, the problem is that we set mSkipDrawing = false on the first text run, in its constructor, before the font group starts loading any fonts. We should be able to ask the font group each time in gfxTextRun::Draw what its mSkipDrawing status is.

Comment 4

[Cameron McCormack (:heycam)]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=1bb239735fe4649cd5466c5743c73283dd0c73f3

Comment 5

[Cameron McCormack (:heycam)]

This is actually a regression from when font-display was originally implemented, in bug 1157064.

Comment 6

[Cameron McCormack (:heycam)]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=df8cc36652d50d1acaa10bc98286993249f78459

Comment 7

[Cameron McCormack (:heycam)]

Attachment: Bug 1639698 - Ensure that the first text run that triggers a font load has its drawing skipped if needed.

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/8fcbd624276b
Ensure that the first text run that triggers a font load has its drawing skipped if needed. r=jfkthame

Comment 9

[Razvan Maries]

https://hg.mozilla.org/mozilla-central/rev/8fcbd624276b

Comment 10

[BugBot [:suhaib / :marco/ :calixte]]

The patch landed in nightly and beta is affected.
:heycam, is this bug important enough to require an uplift?
If not please set status_beta to wontfix.

For more information, please visit auto_nag documentation.

Comment 11

[Cameron McCormack (:heycam)]

Longstanding bug, doesn't need uplift.